A number of people have already posted about using this 3.5 inch display
for their Pi's due to the low cost (£20.02 in the UK, I found if for under $18 from Amazon in the USA).
The big downside is the need for 12V to power the display so here are instruction on how to modify it so that you can power it from USB.
Tools/parts used: Small phillips screwdriver, soldering iron, wire cutters/strippers, USB cable, RCA male plug.
1) Open the display by removing the 4 screws on the back.
2) The insides should look like this:
If not back to the drawing board.
Unplug the video/power cable. Unplug the ribbon cable to the display by sliding the black clip away from the body of the connector. It only moves a few mm, at which point the cable should slide out.
DO NOT remove the black clip from the body of the connector.
3) Remove the two screws holding the board to the front housing.
4) At this point prepare the rear housing:
Take an old USB cable and cut it to the length you want for your power cable.
Drill a hole in the display rear housing between the Menu and - buttons and slightly closer to the display edge. The hold needs to be just wide enough for the cable you have used.
Feed the cable through the hold.
5) Remove the foam pad from the back of the PCB and put it to one side. the Locate and remove the 12V to 5V regulator chip (number 1 in the image below). If you have a soldering heat gun then that is the simplest method, if not then use a soldering iron and small knife or tweezers to lift each pin in turn. Don't worry about damaging the part, we're throwing it away.
6) Sanity check: Make sure your USB cable is threaded through the rear housing before this step or you'll have to re-do it.
Locate the unused 4 pin header on the board close to the power/video in header (number 2 in the image above). Solder the red cable from your USB cord to the hole furthest from the edge (the bottom one in the image). Solder the black cable to the next hole along. (2nd from the bottom in the image.)
It should now look like this
For added strength you can optionally solder the ground from the USB cable to either the same location as the black cable or I found it easier to use one of the ground pins on the switches as shown here:
Trim back the white and green cables and leave them unconnected. Make sure they aren't somewhere that could short to the 5V line.
7) Take the power/video cable that came with the display and cut it just before the junction where it splits.
(hint for people sensible enough to read all the instructions before starting, it's easiest to do this cut earlier, that way you can remove the cable from the housing and keep it from getting in the way).
Sanity check: Make sure the cable is threaded onto the display rear housing and if applicable that the the body of the RCA connector is on the cable before proceeding.
Strip back the cable and solder your male RCA plug on to the end. The yellow wire goes to the middle pin, the black to the outside. The white and red
you can just trim back out of the way.
This does result in a fairly short display cable but for a portable system that is normally what you want.
8) Put the foam back on the rear of the pcb and re-attach it to the front housing. Plug in the display input cable. Check that the clip on the LCD ribbon cable socket is still in the open position (shown in the picture below). Slide the cable in to the socket as far as you can, take extra care that it's in square. Push the clamp down towards the body of the connector to lock the cable in place.
It should now look like this (but with the display cable clamped in)
9) Put the rear housing back on, this is a little tricky due to the extra cables. I found that routing the USB cable between the screw post and the edge of the housing worked well for both keeping it in place and offering some strain relief.
10) Power the display and Pi from a USB power source.
My display looked to take 300mA when running from 5V so if possible budget up to 500mA just to allow for variations. Having said that I plan to run both the display and Raspberry Pi from a single source that can supply up to 1A and don't expect to see any problems.
Any questions feel free to email me.